The present invention relates generally to Radio Frequency (RF) circuits and more particularly relates to an enhanced performance antenna suitable for use with portable wireless devices.
In many wireless devices today, it is either preferable or required that the antenna be incorporated within the housing of the device. This requirement may be dictated by marketing demands, user demands, etc. In most cases, meeting this requirement means that the antenna is placed in close proximity to one or more signal wires in the device. The signal wires form part of the circuitry in the device and typically are used to interconnect internal modules within the device.
A problem, however, of placing the antenna in close proximity to signal wires is that relatively large passive and active interference is likely to be generated resulting in a degradation of system performance. The level of interference generated depends on a number of factors including, for example, the proximity of the signal wires to the antenna, the actual shape and form of antenna used, the particular frequency band used and the characteristics of the signals carried by the wires.
The passive interference is generated as a result of the mutual coupling between the signal wires and the antenna element. This is equivalent to either the shorting of the antenna or a considerable reduction of its impedance. The active interference is a result of the induced noise generated by the wires and fed into the input of the receiver through the antenna. Another source of active interference is the high levels of RF power generated by the antenna during the time it is used to transmit which may interfere with the circuitry connected to the wires. A common means of combating this problem is to place one or more RF chokes on the wires in order to create RF discontinuities.
A block diagram illustrating an antenna in close proximity to a plurality of signal traces is shown in FIG. 1. The RF circuitry board 10 includes an RF generator 12 coupled to an antenna 14. The signal generation circuitry 16 has a plurality of signal wires 18 that run in close proximity to the antenna 14. The antenna and the signal wires are in close proximity when the distance between them is within approximately xc2xc wavelength. For a 1 GHz signal, this corresponds to 7.5 centimeters, assuming the antenna and signal wires are separated by air.
Each of the signal wires has some impedance associated therewith. For example, three wires are shown, one having an inductive impedance 20, capacitive impedance 22 and resistive impedance 24. In addition, the signal wires 18 run substantially parallel with the antenna 14. If the connecting signal wires have relatively low impedance to ground at RF, they will short the antenna thus affecting the radiation pattern of the antenna. Note that this is usually the case since it is common to use decoupling capacitors at the edge of board.
The present invention overcomes the problems and disadvantages of the prior art by providing an enhanced antenna apparatus suitable for use with portable wireless devices. The antenna of the present invention comprises a conductive sleeve having an inner core that can accommodate one or more signal wires passing therethrough. The conductive sleeve functions as the radiating element of the antenna. Passing the signal wires through the conductive sleeve, eliminates the mutual impedance between the antenna itself and signal wires passing through the antenna. In order for the mutual impedance to be eliminated and the benefits of the invention realized, the length of the conductive sleeve must an integer multiple of one half the wavelength of the desired frequency. Note that typically, an integer multiple of one (i.e. xc2xd the wavelength) is used.
Example embodiments of the antenna comprise a sleeve antenna and a printed antenna. In practice, the antenna is connected to an RF generator which may comprise either transmitter, receiver or both as the antenna of the present invention may be used for either transmission or reception.
There is thus provided in accordance with the present invention a radio frequency (RF) antenna comprising an electrically conductive sleeve having a length substantially equal to an integer multiple of one half wavelength of the desired frequency, the conductive sleeve adapted to function as a radiating element of the antenna, wherein the conductive sleeve adapted to peripherally surround one or more signal wires inserted into an inner portion of the sleeve such that the signal wires pass completely through the conductive sleeve and wherein the signal wires appear as an open circuit at both ends of the conductive sleeve thus substantially reducing the mutual impedance between the sleeve and the wires as a consequence of the transmission line effect of the one half wavelength length of the sleeve.
There is also provided in accordance with the present invention a radio frequency (RF) antenna comprising a printed circuit board having a top layer, bottom layer and one or more middle layers situated between the top layer and bottom layer, an elongated electrically conductive pattern printed on the top layer and the bottom layer of the printed circuit board, respectively, the pattern having a length substantially equal to an integer multiple of one half wavelength of the desired frequency, the pattern adapted to function as a radiating element of the antenna, wherein the width of the pattern are sufficiently large enough to cover one or more signal traces printed on the middle layers of the printed circuit board such that the signal traces pass in parallel with and through the length of the pattern and wherein the signal traces appear as an open circuit at both ends of the printed pattern thus substantially reducing the mutual impedance between the pattern and the signal traces as a consequence of the transmission line effect of the one half wavelength length of the printed pattern.
There is further provided in accordance with the present invention a wireless device comprising a housing, a first circuit module including a radio frequency (RF) generator, an electrically conductive sleeve having a length substantially equal to an integer multiple of one half wavelength of the desired frequency, the conductive sleeve connected to the RF generator and adapted to function as a radiating element of an antenna, a second circuit module connected to the first circuit module via one or more connecting signal wires, wherein the conductive sleeve is adapted to peripherally surround the one or more connecting signal wires inserted into an inner portion of the sleeve such that the signal wires pass completely through the conductive sleeve and wherein the signal wires appear as an open circuit at both ends of the conductive sleeve thus substantially reducing the mutual all impedance between the sleeve and the wires as a consequence of the transmission line effect of the one half wavelength length of the sleeve.
There is also provided in accordance with the present invention a method of constructing a radio frequency (RF) antenna, comprising the steps of providing an electrically conductive sleeve having a length substantially equal to an integer multiple of one half wavelength of the desired frequency, the conductive sleeve adapted to function as a radiating element of the antenna, shaping the conductive sleeve so as to peripherally surround one or more signal wires inserted into an inner portion of the sleeve such that the signal wires pass completely through the conductive sleeve and wherein the signal wires appear as an open circuit at both ends of the conductive sleeve thus substantially reducing the mutual impedance between the sleeve and the wires as a consequence of the transmission line effect of the one half wavelength length of the sleeve.